Organic electroluminescence (EL) devices utilizing organic substances are much expected to be useful as inexpensive, large-sized full color display devices of solid state emission type and many developments have been made thereon. An organic EL device is generally constructed from a light emitting layer and a pair of opposite electrodes sandwiching the light emitting layer. When an electric field is applied between the electrodes, electrons are injected from a cathode and holes are injected from an anode. The injected electrons recombine with the injected holes in the light emitting layer to form excited states. When the excited states return to the ground state, the energy is released as light.
As compared with inorganic light emitting diodes, conventional organic EL devices require high driving voltage, exhibit low emission efficiency, and cause significant deterioration in their properties. Therefore, the organic EL device has not yet been put into practical use. Recently, although the properties of organic EL device have been improved gradually, a higher emission efficiency at a lower driving voltage is still required.
To meet such requirement, Patent Document 1 discloses a device which includes a compound having a benzimidazole structure as a light emitting material. It is reported that the device emits at a luminance of 200 cd/m2 under a voltage of 9 V. Patent Document 2 describes a compound having a benzimidazole ring and an anthracene skeleton. Patent Document 3 describes a compound having an imidazole ring, which is used in the light emitting layer and the hole blocking layer. However, it has been still required to develop an organic EL device which can be driven at lower voltage and exhibits a higher emission efficiency as compared with organic EL devices employing the compounds proposed in the patent documents.